Doc:
The more I think about it, the more I think we may both be right. In this area, you need to provide a gravel base under an arena because when the soils get saturated, there is no bottom to them. Once, when my dealer was delivering a piece of equipment, the driver had a small dozer (NH, about D4 size) on his truck. As he was backing out of my driveway, he cut a corner a little sharp and got his wheels off the driveway. The soils were saturated, and he went in up to the axle -- had to jack up the truck enough to level it, unload the Dozer, and use the Dozer to pull him out. As long as he stayed on the driveway, which had a good gravel base, he had no problem.
That's also why folks back here put a gravel base under their arenas. If they don't have a good, solid base (sub-base, if you will), the bottom kind of goes out from under the arena and you develop sinkholes /forums/images/graemlins/shocked.gif. Eventually, the whole arena needs to be redone.
Arena construction varies around the country. In southern Idaho, it seemed all I had to do was drag the riding surface once in awhile, but we only got about 12 inches of rain a year. In Yuma, AZ, (3 inches per year) the soils were so sandy we also just had to drag a level surface. In Reno, NV, (4 inches per year) our soil was DG (Decomposed Granite), which was hard as a rock when dry -- just needed sand on top of it for a cushion for the riding surface. In Virginia, we get 30-40 inches per year, so we need a more solid base and good drainage. Of course, in all cases, I was not trying to create a riding surface that would stand up to heavy use, like a roping or reining arena -- just someplace my wife can train her horses safely.
However, if I get around to stripping off the sand, I will strongly consider an intermediate layer of clay, which would help to hold the gravel, as well. It will mean adding some additional drainage, but that was the cheapest part of the whole project. Thanks for the advice.
Kip